CNC Plasma

with Cut 40

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Back at the end of 2014, I bought a cheap Chinese Plasma Cutter, called a Giant Cut40 from a local trader, at £185, it seemed to be a reasonable buy, the trader was local in case of problems, so it was a reasonable safe purchase.

By January 2015 I was decided it was project time and I started to acquire parts to make a CNC frame for the plasma, Nema 23 stepper motors, timing belts and pulleys were purchased via Ebay along with a ball screw and a 4 channel controller.  The software in use at the time on a CNC milling machine was CNCPro an old DOS based program.

The frame slowly started to take shape over the next few months, I used 50 x 25 mm box section.

By this time it was June 2015, so I was six months into the project and to be honest wondering why I was bothering.

At the end of June 2015, I thought the stage had been reached to move the frame from the floor to a bench position and apart from blowing controllers to useless pieces dead electronics every now and then I managed to achieve some good cuts.  I use Qcad under Linux for my drawing package and purchased a copy of Sheetcam to translate across to Gcode.

The following video was June 2016 so still using the dos program CNCPro But it toom me to this stage to stop the controllers from failing on arc start.  A home made filter system of caps Zener diodes and MOV's between the motors and the controller did the job for the most part.

One of the issues was cutting arc start up.  To this point I was dragging the plasma tip across the metal, that was the only way to ensure the arc started.  But I knew I had to improve things, so that meant investing time in better software such as Mach3 even though Mach4 was coming out soon.

I looked at how I could get some form of pilot arc from the cut 40 so I started to experiment, I did try it in the early days with some success but for some reason stopped, now was time to try again.

So I proved a system that would work and improved on it in stages.  It is worth note that in 2016 and 2017 I started to see Cut 50 Plasma cutters with a pilot arc built in, Oh well!
The next stage was to probe the top of the work piece for Zero height.  If I had pilot arc I did not need to drag the tip any more?  So a micro-switch was fitted to the 'floating head' design I was using, wired into the probe input of mach3 and some macro code worked out with help from the Mach3 forum and away I went.

It was now December 2016, 2 years from buying the Plasma cutter and nearly two years into the project of CNC'ing the unit.  It has been suggested I should have bought a Hypertherm, in fact I still get those comments, perhaps when I can have a table bigger then the size of a desk I may, but for now :)

The next stage of improvement was to add Torch height Control, THC.  Mach3 has a means of taking signals from some basic units and moving the Z axis up or down to suit, but prices were again not cheap and would cost me more that the Plasma cutter cost in the first place, so a sub project was started, please see a little further down.

Filtering on the CNC Plasma

I have had a few comments about filtering, I guess that as people start to build their own units, like me we have issues with the HF start on the Chinese cut machines.

So here is a circuit of the filter unit I added to the output from the motor controller to the stepper motors.  The diagram only shows the output from one motor/axis, so if you use 3 axis, you need 3 filters.  This worked on mine, or shall I say helped a lot, each machine set-up will be different, so best experiment yourself and see what is best for your own machine.

Here is the Youtube video that accompanies the circuit.

A few repairs and a better camera!

June 2017 and I have just updated the table, for the table it is a major update and potentially throws all my previous cut setting out of the window!  The ball screw and number of steps per mm, meant the maximum the table would run the Y axis at reliably was a maximum speed of 1200 mm per minute. I would cut at 900 mm per minute to make sure it did not miss steps.
So the Y axis was changed over to the same belt system as X axis and I could go to a maximum speed of 7,000 mm per minute. (275 ipm).

So  new learning curve awaits as I figure out Amps verses speed on my machine.

Cut parameters

I have been playing again with settings and  on 1.5mm steel cutting at 27 Amps at 2,200 mm/min.  Also a repair, or more like permanent fix of the Z height probe switch as it kept moving!

So once I sorted that and added an aluminium shield to protect the Y ways from the spash-back I had another go I what I call a test piece.  Apart from desperately trying to pick out the tip-ups before they caught on the head, this cut without any errors. total time was something like 18 minutes 15 seconds.

Anyway this was my test piece for tonight, 450mm diameter circle.

I am saving my newer cutting parameters and will update the table when I have played a bit more.

The above mentioned Z Probe switch meant I needed to remove the axis to sort, so I did a couple of videos on my way of doing the Z zero setting. details of mechanical switch,macros and Sheetcam.

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